The lanthanide ions as structural probes in biological and model systems

Kinetic mechanisms by which nickel alters the calcium (Ca2+) transport in intact rat liver

In the present work, the multiple-indicator dilution (MID) technique was used to investigate the kinetic mechanisms by which nickel (Ni²⁺) affects the calcium (Ca²⁺) transport in intact rat liver. ⁴⁵Ca²⁺ and extra- and intracellular space indicators were injected in livers perfused with 1 mM Ni²⁺, and the outflow profiles were analyzed by a mathematical model. For comparative purposes, the effects of norepinephrine were measured. The influence of Ni²⁺ on the cytosolic Ca²⁺ concentration ([Ca²⁺]_c) in human hepatoma Huh7 cells and on liver glycogen catabolism, a biological response sensitive to cellular Ca²⁺, was also evaluated. The estimated transfer coefficients of ⁴⁵Ca²⁺ transport indicated two mechanisms by which Ni²⁺ increases the [Ca²⁺]_c in liver under steady-state conditions: (1) an increase in the net efflux of Ca²⁺ from intracellular Ca²⁺ stores due to a stimulus of Ca²⁺ efflux to the cytosolic space along with a diminution of Ca²⁺ re-entry into the cellular Ca²⁺ stores; (2) a decrease in Ca²⁺ efflux from the cytosolic space to vascular space, minimizing Ca²⁺ loss. Glycogen catabolism activated by Ni²⁺ was transient contrasting with the sustained activation induced by norepinephrine. Ni²⁺ caused a partial reduction in the norepinephrine-induced stimulation in the [Ca²⁺]_c in Huh7 cells. Our data revealed that the kinetic parameters of Ca²⁺ transport modified by Ni²⁺ in intact liver are similar to those modified by norepinephrine in its first minutes of action, but the membrane receptors or Ca²⁺ transporters affected by Ni²⁺ seem to be distinct from those known to be modulated by norepinephrine.

Luminal addition of non-permeant Eu3+ interferes with luminal Ca2+ regulation of the cardiac ryanodine receptor

Dysregulation of the cardiac ryanodine receptor (RYR2) by luminal Ca²⁺ has been implicated in a life-threatening, stress-induced arrhythmogenic disease. The mechanism of luminal Ca²⁺-mediated RYR2 regulation is under debate, and it has been attributed to Ca²⁺ binding on the cytosolic face (the Ca²⁺ feedthrough mechanism) and/or the luminal face of the RYR2 channel (the true luminal mechanism). The molecular nature and location of the luminal Ca²⁺ site is unclear. At the single-channel level, we directly probed the RYR2 luminal face by Eu³⁺, considering the non-permeant nature of trivalent cations and their high binding affinities for Ca²⁺ sites. Without affecting essential determinants of the Ca²⁺ feedthrough mechanism, we found that luminal Eu³⁺ competitively antagonized the activation effect of luminal Ca²⁺ on RYR2 responsiveness to cytosolic caffeine, and no appreciable effect was observed for luminal Ba²⁺ (mimicking the absence of luminal Ca²⁺). Importantly, luminal Eu³⁺ caused no changes in RYR2 gating. Our results indicate that two distinct Ca²⁺ sites (available for luminal Ca²⁺ even when the channel is closed) are likely involved in the true luminal mechanism. One site facing the lumen regulates channel responsiveness to caffeine, while the other site, presumably positioned in the channel pore, governs the gating behavior.

Voltage-gated calcium channels: Determinants of channel function and modulation by inorganic cations

Voltage-gated calcium channels (VGCCs) represent a key link between electrical signals and non-electrical processes, such as contraction, secretion and transcription. Evolved to achieve high rates of Ca(2+)-selective flux, they possess an elaborate mechanism for selection of Ca(2+) over foreign ions. It has been convincingly linked to competitive binding in the pore, but the fundamental question of how this is reconcilable with high rates of Ca(2+) transfer remains unanswered. By virtue of their similarity to Ca(2+), polyvalent cations can interfere with the function of VGCCs and have proven instrumental in probing the mechanisms underlying selective permeation. Recent emergence of crystallographic data on a set of Ca(2+)-selective model channels provides a structural framework for permeation in VGCCs, and warrants a reconsideration of their diverse modulation by polyvalent cations, which can be roughly separated into three general mechanisms: (I) long-range interactions with charged regions on the surface, affecting the local potential sensed by the channel or influencing voltage-sensor movement by repulsive forces (electrostatic effects), (II) short-range interactions with sites in the ion-conducting pathway, leading to physical obstruction of the channel (pore block), and in some cases (III) short-range interactions with extracellular binding sites, leading to non-electrostatic modifications of channel gating (allosteric effects). These effects, together with the underlying molecular modifications, provide valuable insights into the function of VGCCs, and have important physiological and pathophysiological implications. Allosteric suppression of some of the pore-forming Cavα1-subunits (Cav2.3, Cav3.2) by Zn(2+) and Cu(2+) may play a major role for the regulation of excitability by endogenous transition metal ions. The fact that these ions can often traverse VGCCs can contribute to the detrimental intracellular accumulation of metal ions following excessive release of endogenous Cu(2+) and Zn(2+) or exposure to non-physiological toxic metal ions.

Studies on the complexation of Pr(III) and Nd(III) with glycyl-glycine (gly-gly) using spectral analysis of 4f-4f transitions and potentiometric titrations

The interaction of gly-gly with Pr(III) and Nd(III) have been studied in different aquated organic solvents like CH(3)OH, CH(3)CN, dioxane and DMF by using 4f-4f transitions spectra. Various energy interaction parameters like Slater Condon (F(k)), Racah (E(k)), Lande (ξ(4f)), nephelauxetic effect (β), bonding (b(1/2)) and percent covalency (δ) parameters have been calculated to explain the nature of complexation. The intensity parameters like oscillator strength (P) and Judd-Ofelt parameters (T(λ), λ=2, 4, 6) also support the mode of binding of gly-gly to metal ions. The results show that Pr(III) and Nd(III) with gly-gly form complexes by ionic linkages with carboxylate anion with weak covalency. The protonation constants and metal-ligand stability constants have also been determined by potentiometric measurements in aqueous medium at different temperatures (290, 300 and 310 K) at constant ionic strength, 0.1 mol dm(-1). The results show the formation of metal-ligand complexes in the stoichiometric ratio 1:1. The stability of complexes is more in Nd(III) complexes as compared to Pr(III) complexes. Thermodynamic parameters (ΔG, ΔH and ΔS) of complexes were also calculated and the negative values of ΔG and ΔH show that the complex reactions are spontaneous and exothermic. The positive values of ΔS indicate high stability of complex reactions which are enthalpy-driven.

Comparative 4f-4f absorption spectral study for the interactions of Nd(III) with some amino acids: Preliminary thermodynamics and kinetic studies of interaction of Nd(III):glycine with Ca(II)

Spectral analysis of Nd(III) complexes with some amino acids viz.; glycine, l-alanine, l-phenylalanine and l-aspartic acid in the presence and absence of Ca(2+) was carried out in some organic solvents; CH(3)OH, CH(3)CN, DMF and dioxane using comparative absorption spectra of 4f-4f transitions. The study was carried out by evaluating various energy interaction parameters like Slator-Condon (F(k)), Lande factor (ξ(4f)), nephelauxetic ratio (β), bonding parameter (b(1/2)), percent-covalency (δ) by applying partial and multiple regression analysis. The values of oscillator strength (P(obs)) and Judd-Ofelt electric dipole intensity parameter T(λ) (λ=2, 4, 6) for different 4f-4f transitions have been calculated. On analysis of the variation of the various energy interaction parameters as well as the changes in the oscillator strength (P(obs)) and T(λ) values, reveal the mode of binding with the different ligands. Kinetic studies for the complexation of Nd(III):glycine:Ca(II) have also been discussed at different temperatures in DMF medium and from it the values of activation energy (E(a)) and thermodynamic parameters like ΔH°, ΔS° and ΔG° for the complexation are evaluated.

Structural identification of cation binding pockets in the plasma membrane proton pump

The activity of P-type plasma membrane H(+)-ATPases is modulated by H(+) and cations, with K(+) and Ca(2+) being of physiological relevance. Using X-ray crystallography, we have located the binding site for Rb(+) as a K(+) congener, and for Tb(3+) and Ho(3+) as Ca(2+) congeners. Rb(+) is found coordinated by a conserved aspartate residue in the phosphorylation domain. A single Tb(3+) ion is identified positioned in the nucleotide-binding domain in close vicinity to the bound nucleotide. Ho(3+) ions are coordinated at two distinct sites within the H(+)-ATPase: One site is at the interface of the nucleotide-binding and phosphorylation domains, and the other is in the transmembrane domain toward the extracellular side. The identified binding sites are suggested to represent binding pockets for regulatory cations and a H(+) binding site for protons leaving the pump molecule. This implicates Ho(3+) as a novel chemical tool for identification of proton binding sites.

Computation of energy interaction parameters as well as electric dipole intensity parameters for the absorption spectral study of the interaction of Pr(III) with l-phenylalanine, l-glycine, l-alanine and l-aspartic acid in the presence and absence of Ca2+ in organic solvents

Studying the absorption difference and comparative absorption spectra of the interaction of Pr(III) and Nd(III) with l-phenylalanine, l-glycine, l-alanine and l-aspartic acid in the presence and absence of Ca(2+) in organic solvents, various energy interaction parameters like Slater-Condon (F(K)), Racah (E(k)), Lande factor (xi(4f)), nephelauxetic ratio (beta), bonding (b(1/2)), percentage-covalency (delta) have been evaluated applying partial and multiple regression analysis. The values of oscillator strength (P) and Judd-Ofelt electric dipole intensity parameter T(lambda) (lambda = 2, 4, 6) for different 4f-4f transitions have been computed. On analysis of the variation of the various energy interaction parameters as well as the changes in the oscillator strength (P) and T(lambda) values reveal the mode of binding with different ligands.

Block of tetrodotoxin-resistant Na+ channel pore by multivalent cations: gating modification and Na+ flow dependence

Tetrodotoxin-resistant (TTX-R) Na(+) channels are much less susceptible to external TTX but more susceptible to external Cd(2+) block than tetrodotoxin-sensitive (TTX-S) Na(+) channels. Both TTX and Cd(2+) seem to block the channel near the "DEKA" ring, which is probably part of a multi-ion single-file region adjacent to the external pore mouth and is involved in the selectivity filter of the channel. In this study we demonstrate that other multivalent transitional metal ions such as La(3+), Zn(2+), Ni(2+), Co(2+), and Mn(2+) also block the TTX-R channels in dorsal root ganglion neurons. Just like Cd(2+), the blocking effect has little intrinsic voltage dependence, but is profoundly influenced by Na(+) flow. The apparent dissociation constants of the blocking ions are always significantly smaller in inward Na(+) currents than those in outward Na(+) current, signaling exit of the blocker along with the Na(+) flow and a high internal energy barrier for "permeation" of these multivalent blocking ions through the pore. Most interestingly, the activation and especially the inactivation kinetics are slowed by the blocking ions. Moreover, the gating changes induced by the same concentration of a blocking ion are evidently different in different directions of Na(+) current flow, but can always be correlated with the extent of pore block. Further quantitative analyses indicate that the apparent slowing of channel activation is chiefly ascribable to Na(+) flow-dependent unblocking of the bound La(3+) from the open Na(+) channel, whereas channel inactivation cannot happen with any discernible speed in the La(3+)-blocked channel. Thus, the selectivity filter of Na(+) channel is probably contiguous to a single-file multi-ion region at the external pore mouth, a region itself being nonselective in terms of significant binding of different multivalent cations. This region is "open" to the external solution even if the channel is "closed" ("deactivated"), but undergoes imperative conformational changes during the gating (especially the inactivation) process of the channel.

Stabilization of calcium- and terbium-carboxylate bonds by NH...O hydrogen bonds in a mononuclear complex: a functional model of the active site of calcium-binding proteins

Novel benzoic acid ligands with bulky amide groups at the ortho position, 2,6-(MeCONH)(2)C(6)H(3)CO(2)H (1) and 2,6-(t-BuCONH)(2)C(6)H(3)CO(2)H (2), and their tris- and tetrakis(carboxylate) complexes with Ca(II) and Tb(III) ions, (NEt(4))(2)[Ca(II)[O(2)C-2,6-(t-BuCONH)(2)C(6)H(3)](4)] (4), [Tb[O(2)C-2,6-(t-BuNHCO)(2)C(6)H(3)](3)(H(2)O)(3)]] (5), and (NMe)(4)[Tb[O(2)C-2,6-(t-BuNHCO)(2)C(6)H(3)](4)(thf)] (6), were synthesized. The formation of the NH...O hydrogen bonds between the amide NH and carboxylate for 2, (NEt(4))[2,6-(t-BuCONH)(2)C(6)H(3)CO(2)] (3), and 4 was determined by (1)H NMR spectroscopy in solution and in the solid state (CRAMPS, IR). The ligand exchange reactions were attempted between 4 and a large excess of 2,4,6- Me(3)C(6)H(3)CO(2)H in chloroform-d solution; however, exchange reaction did not take place, indicating that the Ca(II) ions bound strongly to the carboxylate in 4. The Ca(II) ion binding properties with the benzoate derivatives were also examined using Tb(III) ion as a fluorescence probe. These results indicate that the NH...O hydrogen bonding between the amide NH and the oxygen atom of the carboxylate contributes to strong Ca(II) binding and prevents the dissociation of the calcium-carboxylate bond. The X-ray structural analyses of these complexes revealed that the NH.O hydrogen-bonded carboxylate ligands prefer the chelate-type coordination and create a mononuclear [Ca(O(2)CR)(4)](2)(-) or [Tb(O(2)CR)(4)](-) core with anionic charge, which is known only in the active site of calcium-binding proteins.

Structure and function of "metalloantibiotics"

Although most antibiotics do not need metal ions for their biological activities, there are a number of antibiotics that require metal ions to function properly, such as bleomycin (BLM), streptonigrin (SN), and bacitracin. The coordinated metal ions in these antibiotics play an important role in maintaining proper structure and/or function of these antibiotics. Removal of the metal ions from these antibiotics can cause changes in structure and/or function of these antibiotics. Similar to the case of "metalloproteins," these antibiotics are dubbed "metalloantibiotics" which are the title subjects of this review. Metalloantibiotics can interact with several different kinds of biomolecules, including DNA, RNA, proteins, receptors, and lipids, rendering their unique and specific bioactivities. In addition to the microbial-originated metalloantibiotics, many metalloantibiotic derivatives and metal complexes of synthetic ligands also show antibacterial, antiviral, and anti-neoplastic activities which are also briefly discussed to provide a broad sense of the term "metalloantibiotics."

Dual actions of lanthanides on ACTH-inhibited leak K(+) channels

Bovine adrenal zona fasciculata cells express background K(+) channels (I(AC) channels) whose activity is potently inhibited by ACTH. In whole cell patch clamp recordings, it was discovered that the trivalent lanthanides (Ln(3+)s) lanthanum and ytterbium interact with two binding sites to modulate K(+) flow through these channels. Despite large differences in ionic radii, these Ln(3+)s inhibited I(AC) channels half-maximally with IC(50) values near 50 microM. In addition, these Ln(3+)s blocked and reversed ACTH-mediated inhibition of I(AC) K(+) channels at similar concentrations. The Ln(3+)s did not alter inhibition of I(AC) by angiotensin II or cAMP. Ln(3+)-induced uncoupling of ACTH receptor activation from I(AC) inhibition was prevented by raising the external Ca(2+) concentration from 2 to 10 mM. The divalent cation Ni(2+) (500 microM) also blocked ACTH-dependent inhibition of I(AC) through a Ca(2+)-sensitive mechanism. The results are consistent with a model in which Ln(3+)s produce opposing actions on I(AC) K(+) currents through two separate binding sites. In addition to directly inhibiting I(AC), Ln(3+)s (and Ni(2+)) bind with high affinity to a Ca(2+)-selective site associated with the ACTH receptor. By displacing Ca(2+) from this site, Ln(3+)s prevent ACTH from binding and accelerate its dissociation. These results identify Ln(3+)s as a relatively potent group of noncompetitive ACTH receptor antagonists. Allosteric actions of trivalent and divalent metal cations on hormone binding, mediated through Ca(2+)-specific sites, may be common to a variety of peptide hormone receptors.

Human ceruloplasmin. Intramolecular electron transfer kinetics and equilibration

Pulse radiolytic reduction of disulfide bridges in ceruloplasmin yielding RSSR(-) radicals induces a cascade of intramolecular electron transfer (ET) processes. Based on the three-dimensional structure of ceruloplasmin identification of individual kinetically active disulfide groups and type 1 (T1) copper centers, the following is proposed. The first T1 copper(II) ion to be reduced in ceruloplasmin is the blue copper center of domain 6 (T1A) by ET from RSSR(-) of domain 5. The rate constant is 28 +/- 2 s(-1) at 279 K and pH 7.0. T1A is in close covalent contact with the type 3 copper pair and indeed electron equilibration between T1A and the trinuclear copper center in the domain 1-6 interface takes place with a rate constant of 2.9 +/- 0.6 s(-1). The equilibrium constant is 0.17. Following reduction of T1A Cu(II), another ET process takes place between RSSR(-) and T1B copper(II) of domain 4 with a rate constant of 3.9 +/- 0.8. No reoxidation of T1B Cu(I) could be resolved. It appears that the third T1 center (T1C of domain 2) is not participating in intramolecular ET, as it seems to be in a reduced state in the resting enzyme.

Characterisation of the divalent cation channels of the hepatocyte plasma membrane receptor-activated Ca2+ inflow system using lanthanide ions

The ability of Gd3+ to inhibit vasopressin-stimulated Ca2+ inflow to hepatocytes was compared with its effect on Mn2+ inflow. In the absence of Gd3+, the stimulation of Mn2+ inflow by vasopressin increased with increasing pH of the extracellular medium. Maximal inhibition of vasopressin-stimulated Ca2+ and Mn2+ inflow by saturating concentrations of Gd3+ was 70 and 30%, respectively. Gd3+ also inhibited thapsigargin-stimulated Ca2+ and Mn2+ inflow with maximal inhibition of 70 and 40%, respectively. It is concluded that vasopressin and thapsigargin each activate two types of Ca2+ inflow processes, one which is sensitive and one which is insensitive to lanthanides. The nature of the pore of the lanthanide-sensitive Ca2+ channel was investigated further using different lanthanides as inhibitors. Tm3+, Gd3+, Eu3+, Nd3+ and La3+ each inhibited vasopressin-stimulated Ca2+ and Mn2+ inflow but had no effect on Ca2+ inflow in the absence of an agonist, or on vasopressin-stimulated release of Ca2+ from intracellular stores. Maximal inhibition of vasopressin-stimulated Ca2+ inflow in the presence of a saturating concentration of each lanthanide ranged from 70-90%. An equation which describes a 1:1 interaction of the lanthanide with a putative binding site in the Ca2+ channel gave a good fit to dose-response curves for the inhibition of vasopressin-stimulated Ca2+ inflow by each lanthanide. Lanthanides in the middle of the series exhibited the lowest dissociation constant (Kd) values. The Kd for Gd3+ increased with increasing extracellular Ca2+ concentration, suggesting competitive inhibition of Ca2+ binding by Gd3+. In the absence of lanthanide, vasopressin-stimulated Mn2+ inflow was substantially reduced when the plasma membrane was depolarised by increasing the extracellular K+ concentration. Changing the membrane potential had little effect on the maximum inhibition by Gd3+ of vasopressin-stimulated Mn2+ inflow. The Kd for inhibition of vasopressin-stimulated Ca2+ inflow by Gd3+, measured at the lowest attainable membrane potential, was about 6-fold lower than the Kd measured at the highest attainable membrane potential. The idea that there is a site in the vasopressin-stimulated lanthanide-sensitive Ca2+ channel composed of carboxylic acid groups which bind Ca2+, Mn2+ or a lanthanide ion is consistent with the data obtained using the different lanthanides.

Probing the metal-binding sites of cod parvalbumin using europium(III) ion luminescence and diffusion-enhanced energy transfer

Excitation spectroscopy of the 7F0----5D0 transition of Eu3+ and diffusion-enhanced energy transfer are used to study metal-binding characteristics of the calcium-binding protein parvalbumin from codfish. Energy is transferred from Eu3+ ions occupying the CD- and EF-binding sites to the freely-diffusing Co(III) coordination complex energy acceptors: [Co(NH3)6]3+, [Co(NH3)5H2O]3+, [CoF(NH3)5]2+, [CoCl(NH3)5]2+, [Co(NO2)3(NH3)3], and [Co(ox)3]3-. In the absence of these inorganic energy acceptors, the excited-state lifetimes of Eu3+ bound to the CD and EF sites are indistinguishable, even in D2O; however, in the presence of the positively charged energy acceptor complexes, the Eu3+ probes in the cod parvalbumin have different excited-state lifetimes due to a greater energy-transfer site from Eu3+ in the CD site than from this ion in the EF site. The observation of distinct lifetimes for Eu3+ in the two sites allows the study of the relative binding site affinities and selectivity, using other members of the lanthanide ion series. Our results indicate that during the course of a titration of the metal-free protein, Eu3+ fills the two sites simultaneously. Eu3+ is competitively displaced by other Ln3+ ions, with the CD site showing a preference for the larger Ln3+ ions while the EF site shows little, if any, competitive selectivity across the Ln3+ ion series.

A possible calcium binding site in animal lectins: a 1H-NMR study of the interaction between lanthanides and a synthetic peptide from a highly conserved domain of Pleurodeles lectin

1H-NMR techniques have been used to study the metal binding properties of a synthetic peptide of 15 amino acids corresponding to a highly conserved domain of Pleurodeles lectin. The addition of lanthanum chloride or praseodymium chloride in a peptide solution induces some conformational changes as displayed by several concerted variations of peptide resonances. The Ln3+ concentration dependence of the chemical shifts was used to calculate the Ln3+ binding constants. The dissociation constants of 95 microM and 280 microM were found for La3+ and Pr3+, respectively.

Characterization of cationic binding sites of neurotoxins from venom of the scorpion (Centruroides sculpturatus Ewing) using lanthanides as binding probes

Binding sites for cations were probed in the structures of protein neurotoxins from Centruroides sculpturatus by enhancement of terbium(III) fluorescence, detected by emission at 552 nm, when aromatic side-chains of the toxins were activated at 286 nm. Gadolinium, Gd(III), was used as a cation binding probe by observing its effects on nuclear magnetic resonance (NMR) spectra. Toxins CsE-v2 and v3, when bound to Tb(III), enhance luminescence of Tb(III) 20-fold whereas CsE-v1 enhances Tb(III) luminescence about 15-fold. Toxins CsE-I and V have no effect on the luminescence of Tb(III) implying that these latter two toxins have structures incompatible with efficient energy transfer from activated aromatic side-chains. Enhancement of fluorescence is pH dependent and is competitively inhibited by alkaline earth divalent cations and by other lanthanide(III) ions. Neodymium, Nd(III), with an ionic radius of 0.995 A is the most efficient of the lanthanide ions and the divalent cations in displacement of Tb(III) from the toxins. Relaxation enhancements of aromatic CH resonances by Gd(III) are apparent with tyrosines 4, 42, 38, 14-40 peak and tryptophan 47. Results from pH vs fluorescence studies suggest that carboxyl groups are involved in binding of Tb(III). Association constants (Ka) of the Tb(III)-CsE-v2 and v3 complexes are respectively 2.5 x 10(3) and 2.4 x 10(3) M-1 determined by fluorescence enhancement and 2.4 x 10(3) and 2.3 x 10(3) M-1 by equilibrium dialysis. Similarly Ka values for toxins CsE I and V are respectively 1.9 x 10(3) and 1.8 x 10(3) M-1 determined by equilibrium dialysis. Experimental evidence suggests that at least two Tb(III)s are bound per toxin molecule. The results from these studies are discussed in relation to the tertiary structure of toxin CsE-v3.

The transverse location of the retinal chromophore in the purple membrane by diffusion-enhanced energy transfer

We have used fluorescence energy transfer in the rapid-diffusion limit (RDL) to estimate the trans-membrane depth of retinal in the purple membrane (PM). Chelates of Tb(III) are excellent energy donors for the retinal chromophore of PM, having a maximum Ro value for Förster energy transfer of approximately 62 A (assuming a donor quantum yield of 1). Energy transfer rates were measured from the time-resolved emission kinetics of the donor. The distance of closest approach between chelates and the chromophore was estimated by simulating RDL energy-transfer rate constants according to geometric models of either PM sheets or membrane vesicles. The apparent rate constant for RDL energy transfer between Tb(III)HED3A and retinal in PM sheets is 1.5(+/- 0.1) x 10(6) M-1 s-1, corresponding to a depth of approximately 10 +/- 2 A for the retinal chromophore. Cell envelope vesicles (CEVs) from Halobacterium halobium were studied by using RDL energy transfer to assess the proximity of retinal to either the extracellular or intracellular face of the PM. The estimated depth of retinal from the extravesicular face of the PM is 10 +/- 3 A, based on the RDL energy-transfer rate constant. Energy-transfer levels to retinal in the PM were estimated by an indirect method with energy donors trapped in the inner-aqueous space of CEVs. The rate constants derived for this arrangement are too low to be consistent with the shortest depth of retinal deduced for PM sheets. Thus, the intravesticular face of CEVs, corresponding to the cytoplasmic face of cells, is the more distant surface from the chromophore of bacteriorhodopsin.

Evidence of a calcium-induced structural change in the ATP-binding site of the sarcoplasmic-reticulum Ca2+-ATPase using terbium formycin triphosphate as an analogue of Mg-ATP

Terbium ions and terbium formycin triphosphate have been used to investigate the interactions between the cation and nucleotide binding sites of the sarcoplasmic reticulum Ca2+-ATPase. Three classes of Tb3+-binding sites have been found: a first class of low-affinity (Kd = 10 microM) corresponds to magnesium binding sites, located near a tryptophan residue of the protein; a second class of much higher affinity (less than 0.1 microM) corresponds to the calcium transport sites, their occupancy by terbium induces the E1 to E2 conformational change of the Ca2+-ATPase; a third class of sites is revealed by following the fluorescence transfer from formycin triphosphate (FTP) to terbium, evidencing that terbium ions can also bind into the nucleotide binding site at the same time as FTP. Substitution of H2O by D2O shows that Tb-FTP binding to the enzyme nucleotide site is associated with an important dehydration of the terbium ions associated with FTP. Two terbium ions, at least, bind to the Ca2+-ATPase in the close vicinity of FTP when this nucleotide is bound to the ATPase nucleotide site. Addition of calcium quenches the fluorescence signal of the terbium-FTP complex bound to the enzyme. Calcium concentration dependence shows that this effect is associated with the replacement of terbium by calcium in the transport sites, inducing the E2----E1 transconformation when calcium is bound. One interpretation of this fluorescence quenching is that the E1----E2 transition induces an important structural change in the nucleotide site. Another interpretation is that the high-affinity calcium sites are located very close to the Tb-FTP complex bound to the nucleotide site.

Interactions between alpha-latrotoxin and trivalent cations in rat striatal synaptosomal preparations

The interactions between alpha-latrotoxin (alpha-LTx), a neurosecretagogue purified from the venom of the black widow spider, and the trivalent cations Al3+, Y3+, La3+, Gd3+, and Yb3+ were investigated in rat striatal synaptosomal preparations. All trivalent cations tested were inhibitors of alpha-LTx-induced [3H]dopamine [( 3H]DA) release (order of potency: Yb3+ greater than Gd3+ approximately Y3+ greater than La3+ greater than Al3+). Only with Al3+ could inhibition of [3H]DA release be attributed to a block of 125I-alpha-LTx specific binding to synaptosomal preparations. The inhibitory effect of trivalent ions was reversible provided synaptosomes were washed with buffer containing EDTA. Trivalent ions also inhibited alpha-LTx-induced [3H]DA release at times when alpha-LTx-stimulated release was already evident. alpha-LTx-induced synaptosomal membrane depolarization was blocked by La3+, but not affected by Gd3+, Y3+, and Yb3+. alpha-LTx-stimulated uptake of 45Ca2+ was inhibited by all trivalent cations tested. These results demonstrate that there exist at least three means by which trivalent cations can inhibit alpha-LTx action in rat striatal synaptosomal preparations: (1) inhibition of alpha-LTx binding (Al3+); (2) inhibition of alpha-LTx-induced depolarization (La3+); and (3) inhibition of alpha-LTx-induced 45Ca2+ uptake (Gd3+, Y3+, Yb3+, La3+).

Lanthanide-stimulated glucose and proline transport across rabbit intestinal brush-border membranes

Trivalent cations of the lanthanide series (La3+----Yb3+) stimulated uptake of proline or glucose in rabbit small intestinal brush-border membrane vesicles. The lanthanides stimulated uptake to an extent greater than Al3+, choline, and in many cases, Na+. A time-course of Er3+-stimulated glucose uptake gave initial rates and overshoots greater than Na+ stimulation. The best activators were Sm3+, Eu3+ and Tm3+, which stimulated proline initial uptakes by 400-600%, and stimulated glucose uptake by 120-150%, compared to Na+. The best lanthanide cotransport activators possessed high third ionization potentials.

Estimation of apparent quadrupolar coupling constants for complexes of magnesium ions with mono- and dicarboxylic acid ligands. Applications to magnesium ion: protein interactions

25Mg+2 ion NMR studies of complexes of magnesium ions with acetate and malonate ligands have yielded apparent quadrupolar coupling constants, chi, of approximately 1.5 MHz. The aquo magnesium ion yields a smaller chi value of 0.12 MHz, consistent with its expected higher symmetry. chi values for magnesium ion: acetate and magnesium ion: malonate complexes are utilized to calculate observed linewidths for magnesium ion: bovine prothrombin fragment 1 and magnesium ion: human Factor XII interactions. These calculated values are compared with observed values and implications of the agreement are discussed.

Complete amino acid sequence of plastocyanin from a green alga, Enteromorpha prolifera

The complete amino acid sequence of the plastocyanin from the green alga Enteromorpha prolifera has been determined by Edman degradation of the intact molecule and fragments produced by enzymatic cleavage of the polypeptide chain with chymotrypsin, Staphylococcus aureus protease, proline-specific endopeptidase, Lys-C endopeptidase and trypsin. The molecule consists of 98 amino acid residues with a calculated relative molecular mass of 10103. The amino acid sequence of E. prolifera plastocyanin shows a high degree of homology with those plastocyanins from other algae and higher plants. In particular, the four residues which are copper ligands in other plastocyanins and in the bacterial electron transport protein azurin (two histidines, one cysteine and one methionine) are conserved. Five out of the six acidic amino acid side-chains which create an 'acidic patch' on the surface of plastocyanin from Populus nigra var. italica [Colman, P. M. et al. (1978) Nature (Lond.) 272, 319-324] are conserved in the amino acid sequence of E. prolifera plastocyanin.

Site-selective laser spectroscopy of lanthanide-binding sites in calmodulin

Site-selective laser spectroscopy has been used to resolve the spectral features of lanthanide fluorescence probe ions in calcium-binding proteins. The capabilities and characteristics of this technique are studied using bovine brain calmodulin where the calcium-binding sites are very similar. Two distinct spectral features are identified. These features were followed during a Eu3+ titration and were found to fill successively, showing they correspond to the high- and low-affinity sites. One set of spectral features is assigned to domains I and III, which are the high-affinity domains, while the other set is assigned to domains II and IV. Additional nonspecific binding is observed after the domains are filled. Tb3+ titrations confirmed earlier results that the tyrosine-containing domains fill second and third (R. W. Wallace, E. A. Tallant, M. E. Duckter, and W. Y. Cheung, 1982, J. Biol. Chem. 257(4), 1845-1854). Site-selective laser spectroscopy was also used to identify the presence of ethylene glycol bis(beta-aminoethyl ether) N,N'-tetraacetic acid contamination that could cause interference in titrations.

Characterization of the internal calcium(II) binding sites in dissolved insulin hexamer using europium(III) fluorescence

The fluorescence of Eu(III) is used to study the nature of the Ca(II) binding sites in the central cavity of the two-zinc(II) insulin hexamer. The dependence of the Eu(III) fluorescence lifetime upon Eu(III) stoichiometry indicates that there are three identical Eu(III) binding sites present in the two-zinc(II) insulin hexamer in solution. Addition of excess Ca(II) causes a decrease in the Eu(III) fluorescence intensity, confirming that Ca(II) competes for the observed Eu(III) sites. The solvent dependence of the Eu(III) fluorescence lifetime (H2O vs. D2O) indicates that four OH groups are coordinated to each Eu(III) in the hexamer. Substitution of Co(II) for Zn(II) causes a decrease in the Eu(III) fluorescence lifetime. Calculations based on Förster energy-transfer theory predict that the Co(II) [or Zn(II) in vivo] and Eu(III) [or Ca(II) in vivo] binding sites are separated by 9.6 +/- 0.5 A. Variation of the metal stoichiometries indicates that all three Eu(III) [or Ca(II) in vivo] sites are equidistant from the Zn(II) sites. We conclude that these sites are identical with the three central Zn(II) sites present in insulin hexamer crystals soaked in excess Zn(II) [Emdin, S. O., Dodson, G., Cutfield, J. M., & Cutfield, S. M. (1980) Diabetologia 19, 174-182] and suggest that these central sites are occupied by Ca(II) in vivo.